This project entails continuing analyses of data derived from a large- scale, placebo-controlled field trial of B subunit-killed whole cell (BS- WC) and killed whole cell-only (WC) oral vaccines against cholera, conducted between 1985-90 in the Matlab field studies area of the International Center for Diarrheal Disease Research, Bangladesh. The trial enrolled ca. 89,000 subjects for study. Analyses of the first three years of follow-up revealed that each vaccine conferred ca. 50% protection against cholera episodes detected among patients seeking care at medical facilities. During the past year, a paper on the fourth and fifth years of surveillance, during which no further protection was evident, was prepared. In addition, an analysis of serum immune responses to V. cholerae 01 among vaccine failures revealed that such vaccinees manifested poor responses, suggesting that impaired immunity may play a role in instances of vaccine failure. During the past year, a large-scale case-control study was conducted to determine whether infection by Helicobacter pylori, a known cause of persistent hypochlorhydria, is a risk factor for cholera in Bangladesh. Preliminary analyses indicate that the risk of clinically severe cholera is elevated in H. pylori-infected individuals, but only when such individuals lack antecedent vibriocidal immunity to cholera. Analyses are now being done to determine whether infection by H. pylori was also a determinant of vaccine failure in the field trial.